MS-102 Deploy and manage a Microsoft 365 tenant Practice Question
An organization has just signed up for Microsoft 365 E3 with the initial domain 'contoso.onmicrosoft.com'. They need to create the first user accounts. What will be the default email address format for these new users if no custom domain is added yet?
⚠ Common exam trap
Many exam-takers assume the email address will automatically match the organization's public domain name (e.g., contoso.com) without realizing that a custom domain must be explicitly added and verified in the Microsoft 365 admin center before it can be used for user email addresses.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
user@contoso.onmicrosoft.com
When a new Microsoft 365 tenant is created with the initial domain 'contoso.onmicrosoft.com' and no custom domain has been added, the default email address format for new users is user@contoso.onmicrosoft.com. This is because the onmicrosoft.com domain is the default tenant domain provisioned by Microsoft Entra ID, and all user principal names (UPNs) and email addresses are automatically assigned this suffix until a custom domain is verified and set as the primary domain.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
user@contoso.onmicrosoft.com
Why this is correct
The initial domain created when a Microsoft 365 tenant is provisioned always uses the <tenantname>.onmicrosoft.com namespace, and contoso.onmicrosoft.com is the default UPN and email domain for all new users. This domain is automatically reserved for your tenant and cannot be used by any other tenant, so assigning user@contoso.onmicrosoft.com is the only valid option listed without additional configuration.
- ✗
user@contoso.com
Why it's wrong here
user@contoso.com would require a custom domain that you own and have registered as contoso.com, then added to the Microsoft 365 tenant and verified via DNS (usually a TXT record) before it can be used in user principal names. Since the sign-up only created the initial onmicrosoft.com tenant, no custom domain is automatically available, and until contoso.com is verified and optionally set as the default domain, new users cannot be assigned that email address.
- ✗
user@microsoft.com
Why it's wrong here
user@microsoft.com is invalid because microsoft.com is a Microsoft-owned corporate domain, not a domain registered to your tenant, and Microsoft 365 only allows user principal names to use domains that are verified as belonging to your organization. Attempting to assign a UPN with microsoft.com would fail during user creation because the domain is not in the tenant's accepted domains list, and mail routing to that address would conflict with Microsoft's own infrastructure.
- ✗
user@contoso.microsoft.com
Why it's wrong here
user@contoso.microsoft.com is not a valid default domain format because Microsoft 365 only provisions the <tenantname>.onmicrosoft.com suffix, and microsoft.com cannot be used as a parent domain to create customer subdomains. You can add custom third-level domains like sub.contoso.com if you own them, but contoso.microsoft.com is under a Microsoft-owned domain that no customer can verify or claim, so it will never appear in the accepted domains list.
Go deeper
Related to this question
Learn chapter
Entra Connect Sync Rules and Filtering
Key term
Microsoft Entra ID
Microsoft Entra ID is a cloud-based identity and access management service that lets employees sign in and access resources both inside and outside of your organization.
Key term
Microsoft 365
Microsoft 365 is a subscription-based cloud service from Microsoft that combines productivity tools like Office apps with security, device management, and online storage.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This MS-102 practice question is part of Courseiva's free Microsoft certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the MS-102 exam.